The present invention relates to novel methods of producing fertilizer products, and more specifically to methods of producing individual granules of fertilizer consisting essentially of a mixture of ammonium sulfate crystals and sewage sludge solids.
It has been recognized that useful fertilizer products may be provided in the form of solid granules consisting essentially of sewage sludge solids and fertilizer salts. Such a product results, for example, from the process described in applicant's prior U.S. Pat. No. 4,038,180 which is primarily concerned with more efficient methods of dewatering sludge to simplify handling and disposal, as well as to provide useful applications thereof. According to this method, wet sludge is mixed with concentrated acid and the resulting mixture of sludge solids and water-diluted acid is mixed with a base, such as ammonia. The heat of the acid-base reaction is utilized to drive off some or all of the water initially contained in the sludge as a granulated product is formed consisting of the salt resulting from the acid-base reaction and the sludge solids remaining after the water has been driven off. It is necessary, of course, to the practise of this method, that the wet sludge and acid be thoroughly mixed prior to contacting the solution with the base to initiate the reaction forming the salt.
Another method of producing a granular fertilizer product comprising a mixture of sewage sludge solids and fertilizer salts is described in U.S. Pat. No. 3,050,383 of J. F. Wilson. According to this method the sewage sludge is previously dried to provide sludge solids having a very low moisture content (3.2% or less) which are then combined in a water solution with the acid and base. Although this method has no utility in the dewatering of sewage sludges, which is the principal thrust of applicant's above-mentioned prior patent, the two methods will apparently produce the same or very similar granular fertilizer products.
Salts produced in acid-base reactions are normally in crystalline form, as opposed to the more desirable spherical or semispherical granular form when the salts are to be employed in solid fertilizer applications. Both of the prior art methods discussed above are able to achieve granulation of the end product with the aid of sludge solids which are present during the acid-base reaction. However, neither provides a process for aiding in the granulation, i.e., size and shape improvement, of salts which have already been formed in the crystalline state.